1. Field of the Invention
The present invention relates to a holder for a drive piston of a setting tool.
2. Description of the Prior Art
European Publication EP-O 346275 B1 discloses an explosive powder charge-operated setting tool including a piston guide and a drive piston displaceable in the piston guide. The piston guide has radial openings facing the drive piston, and spring-biased braking balls extending through the opening and engaging the drive piston. The spring, which applies a biasing force to the braking balls is formed as a ring spring for applying a radially acting, with respect to the piston, biasing force to the braking balls. The ring spring is provided on its inner profile with a bearing surface acting on the braking ball. The bearing surface is inclined to the piston at an acute angle that opens in a direction opposite a setting direction. When the drive piston moves in the setting direction, it entrains the braking balls therewith. The braking balls expand the ring spring, which results in the bearing surface transmitting the radial biasing force to the braking balls. The braking balls are pressed radially against the piston body by the ring spring washer. Even with a small displacement of the drive piston in a direction opposite the setting direction, the braking effect can be substantially reduced or eliminated, as the braking balls displace in the same direction as the drive piston, unloading the ring spring. After being unloaded, the ring spring does not press any more the braking balls against the piston body. Further, a possibility still remains that the drive piston would be displaced, before ignition or firing of the setting tool, in the setting direction as a result of, e.g., the setting tool being pressed hard against a constructional component. The displacement in the return direction is effected due to cooperation of the ring spring with the braking balls.
U.S. Pat. No. 4,162,033 discloses a setting tool with a braking element that continuously applies a braking force to the drive piston.
An object of the present invention is to provide a piston holder having a simplified design and which would reliably retain the drive piston in its ignition-ready position in the absence of ignition.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a piston bolder for the drive piston of a setting tool and including braking means self-lockingly supportable on a drive piston body; and spring means for applying different biasing forces to the braking means in respective opposite directions corresponding to a direction of displacement of the drive piston for reducing self-locking forces with which the braking means is supported on the drive piston body. The biasing forces, which are applied to the drive piston body, only slightly increase when the drive piston is displaced in the setting direction, i.e., toward the mouth of the setting tool for driving in, e.g., a fastening element. The setting process is effected under a relatively large braking force applied by the braking means. Therefore, the drive piston-displacing force should be large enough to overcome the braking force. During the return movement of the drive piston into its ignition-ready position, the self-locking force, with which the braking means is supported on the piston body, should be substantially reduced to insure that the drive piston always reaches its ignition-ready position. Nonetheless, during the return movement of the drive piston, the self-locking force is not eliminated completely, as the drive piston, after reaching its ignition-ready position, should be retained therein by the braking means to prevent the drive piston displacement from its ignition-ready position in the absence of the ignition.
According to an advantageous embodiment of the present invention the braking means is formed as a washer having a central opening for receiving the drive piston body and arranged between two, spaced from each other, axial stops fixed relative to the setting tool. The spring means includes two springs having different spring rates, respectively, and acting, respectively, on opposite circumferential sections and on opposite sides of the washer. The two springs can be formed as helical compression springs or as rubber springs. The washer can be formed as a hardened steel washer. According to the present invention, the washer is eccentrically loaded by the two springs in each operational position of the piston holder in such a way that it is self-locked on the piston body. The functioning of the piston holder is substantially the same in both directions of movement of the drive piston, i.e., when it moves in the setting direction and when it moves back to its ignition ready position. During its movement in the setting direction, the drive piston entrains the washer therewith until the washer hits the stop. At this moment, the self-locking of the washer on the piston body ends. In this position of the washer, only one spring acts on the washer for tilting the same. The biasing force of this spring determines the friction force between the washer and the piston body. Due to the different spring rates of the two springs, different braking forces are applied to the drive piston during movement of the drive piston in the different directions. The braking force, which is applied to the drive piston during its displacement in the setting direction, can be very high. However, the braking force, which is applied to the drive piston during its displacement to its initial, ignition-ready position should be small, but not zero. This is because, on one hand, the drive piston should be able to easily return to its ignition-ready position and, on the other hand, be retained in its ignition-ready position.
According to another advantageous embodiment of the present invention, the braking means includes two, spaced from each other, flanges supported on the drive piston shaft and arranged between two, spaced from each other axial stops fixed relative to the setting tool, and a drag hose expandable in an axial direction of the drive piston body and connected to the two flanges. The spring means includes a first compression spring arranged between one side of one of the flanges and a facing the one side, side of another of the flanges, and a second compression spring arranged between the one side of the one of the flanges and a stop facing the one side of the one of the flanges.
The drag hose is a hose the diameter of which is decreased upon application of a tensioning force thereto, and the diameter of which increases upon application of a compression force thereto. Such a drag hose can be, formed, e.g., of wire coils and cross-weaved. When such a hose surrounds the piston body, it becomes self-locked upon application of a tensioning force thereto. When compressed, the hose releases the piston body to a lesser and greater degree. At its opposite ends, the hose is connected with the two flanges. The weaker, first compression spring is, as discussed above, arranged between the two flanges and provides a minimal friction force. During the displacement of the drive piston in the drive-out or setting direction, the drive piston entrains the drag hose therewith until the front, in the setting direction flange, abuts a stationary with respect to the setting tool, stop. This causes compression of the drag hose, and its self-locking on the piston body is released. The drive piston, during its subsequent movement in the setting direction, simply slides through the hose. The friction force between the piston body and the drag hose is determined by the sum of the biasing forces of the first and second springs. The spring rate of the second spring is greater than that of the first spring. Therefore, the friction force, before release of the self-locking of the hose, during the drive piston movement in the setting direction, is relatively large. Analogous action takes place during the return movement of the drive piston. However, in this case, the hose is extended only by the force generated by the smaller spring. Therefore, the friction between the hose and the drive piston body is small. However, the friction force should be greater than zero as a friction or clamping force should be applied to the drive piston in its ignition-ready position.
Both embodiments of the piston holder according to the invention provide for obtaining of a relatively large holding force. This force insures retaining the drive piston in its ignition-ready position, e.g., in the gas-operated tools, until the combustion is completed.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.